Polypropylene resins have been extensively used in automotive applications and in various fields such as electrical appliances, miscellaneous goods or films, because the articles molded therefrom are excellent in rigidity and high-temperature rigidity. However, conventional polypropylene resins have been limited in the use range, since the articles molded therefrom are deficient in impact resistance.
As typical polypropylene resins having improved impact resistance are known a polypropylene resin comprising propylene/ethylene block copolymers wherein propylene and ethylene are block copolymerized, and polypropylene resin compositions wherein polypropylene is incorporated with an ethylene/.alpha.-olefin copolymer rubber. Since such polypropylene resins have lower rigidity than a propylene homopolymer, these resins are incorporated with inorganic fillers such as talc to make up for lowered rigidity, as the occasion demands, and used mainly as molding materials for exterior automotive trim parts including automotive bumpers and interior automotive trim parts including instrument panels, console boxes, resin pillars and trims. However, inorganic fillers should be incorporated in a large amount so as to enhance the rigidity of the resin composition, which leads to a lowered flowability of the resultant composition and an increased specific gravity of the resultant molded article, resulting in an increased weight of the products.
With a view to improving the impact resistance and stress-whitening resistance of .alpha.-olefin polymers such as polypropylene and polyethylene, the resin compositions wherein hydrogenated diene copolymers are incorporated therein have been proposed.
For example, JP-B-59-37294 discloses a resin compositions wherein a hydrogenated 1,2-polybutadiene is incorporated in polymers or copolymers of .alpha.-olefin with 2 to 20 carbons, especially ethylene homopolymer, propylene homopolymer or propylene/ethylene copolymer.
JP-B-62-45883 and JP-A-4-342752 disclose a resin composition wherein a hydrogenated diblock copolymer of polybutadienes consisting of 1,2-block and 1,4-block is incorporated in .alpha.-olefin polymers or copolymers, in particular polypropylene.
JP-A-5-132606 discloses a resin composition wherein a hydrogenated product of polybutadiene/conjugated diene block copolymers is incorporated in a crystalline propylene/ethylene block copolymer. JP-A-1-168743 and JP-A-1-168744 disclose a resin composition wherein a hydrogenated product of isoprene/butadiene copolymers is incorporated in polyolefins. These resin compositions are improved in both stress-whitening resistance and impact resistance, but remarkably lowered in rigidity and high-temperature rigidity. Thus a practical use of such resin compositions is limited to the field of soft materials.
In order to enhance the reduced rigidity and high-temperature rigidity in the above-described resin compositions wherein the hydrogenated diene block copolymers are incorporated in the propylene/ethylene block copolymers, it is considered that high stereoregular polypropylenes disclosed in JP-A-1-254706 and JP-A-62-187707 may be used instead of said propylene/ethylene block copolymers, whereby the rigidity and the high-temperature rigidity of the molded articles are improved without any increase in the specific gravity. However, a reduction in impact resistance of the molded articles is also anticipated.
As mentioned above, the impact resistance of the polypropylene resin compositions is improved by the incorporation of the ethylene/.alpha.-olefin copolymer rubber or the hydrogenated polybutadiene polymer or copolymer. However, the incorporation of these rubber components in a large amount brings about another problem of lowering the rigidity, the high-temperature rigidity and the flowability of the resulting molded articles. Thus, it has been desired to develop a polypropylene resin composition capable of improving the impact resistance of the molded articles without reducing the rigidity and the high-temperature rigidity of the molded articles as well as the flowability of the resin compositions.
Accordingly, an object of the present invention is to provide a polyolefin resin composition having good flowability of the resin composition, good rigidity, high-temperature rigidity and impact resistance of the molded article as well as a well-balanced combination of these properties.